Medical stents for body lumens exhibiting peristaltic motion

ABSTRACT

A stent for reinforcement of the lumen of a peristaltic organ is formed by knitting preferably a nitinol wire into a pattern of overlapping loops selected such that from a relaxed state each row of loops may shift axially relative to and independently of the rows on either side. This local lengthening and shortening accommodate peristalsis of the organ without migrating within the organ. A stent is also shown which comprises two resilient cylindrical mesh layers and a semi-permeable compliant membrane such as expanded polytetrafluoroethylene, sandwiched between. The two mesh layers may be knit of a flexible filament, and the knit may be configured so that the stent can adapt to peristalsis of the body lumen. A method is also shown of manufacturing a delivery system for a resilient tubular device such as a stent so that the device can be inserted into the body in a substantially reduced diameter. The method uses a confining block having a bore and a slot leading into the bore. The tubular device is pinched and inserted into the bore and the slot, two mandrels are inserted into the bore, one inside the device and one outside and the mandrels are revolved about each other to roll the device on itself.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/528,061,filed Sep. 14, 1995, now U.S. Pat. No. 5,662,713, which is acontinuation of application Ser. No. 07/960,584, filed Oct. 13, 1992,now abandoned, which is a continuation-in-part of application Ser. No.07/773,847, filed Oct. 9, 1991, now U.S. Pat. No. 5,234,457.

FIELD OF THE INVENTION

This invention relates to endoprosthetic stents that are placed withinbody lumens that exhibit physiologic motion such as peristaltic motion.

BACKGROUND OF THE INVENTION

Medical stents are tubular endoprostheses placed within the body toperform a function such as maintaining open a body lumen, for example, apassageway occluded by a tumor. Typically, the stent is delivered insidethe body by a catheter that supports the stent in a compacted form as itis transported to the desired site. Upon reaching the site, the stent isexpanded so that it engages the walls of the lumen. The expansionmechanism may involve forcing the stent to expand radially outward, forexample, by inflation of a balloon carried by the catheter, toinelastically deform the stent and fix it at a predetermined expandedposition in contact with the lumen wall. The expansion balloon can thenbe deflated and the catheter removed.

In another technique, the stent is formed of a highly elastic materialthat will self-expand after being compacted. During introduction intothe body, the stent is restrained in the compacted condition. When thestent has been delivered to the desired site for implantation, therestraint is removed, allowing the stent to self-expand by its owninternal elastic restoring force.

Strictures of the esophagus often produce obstructive dysphagiaresulting in debilitating malnutrition. To date, the theoreticaladvantages of placing a plastic stent to restore the patient's abilityto swallow have been offset by technical difficulty of placement,morbidity and mortality associated with the procedure, and poorlong-term prosthesis performance. In particular, previous stents havetransmitted the force and deformation of peristaltic wavesinappropriately, for instance causing the stent to creep toward thestomach, perforate the esophagus, or rupture the aorta.

SUMMARY OF THE INVENTION

In a first aspect, the invention features a method for providingreinforcement to the lumen of a peristaltic organ. The stent is formedby knitting a filament into interknit loops, the pattern of the loopsselected such that from a relaxed state each row of loops may shiftaxially relative to and independently of the rows on either side. Thelocal lengthening and shortening allowed by the shifting allows thestent to accommodate the peristalsis of the organ without migratingwithin the organ.

Preferred embodiments of the stent feature the following. The lumentreated is the esophagus. The elongation factor ε by which the stent canlocally lengthen by shifting is related to the angle Θ at which thelumen can incline inward by the relationship: ε=1.0/cos Θ. The stent isknitted of metal wire to be self-expandable such that the stent expandsoutward against the body lumen wall by an elastic restoring force of thewire. The stent is knitted from nitinol wire having a diameter of about0.15 mm. The stent, in its free state, has a point of constrictedcross-section. The constriction may have a valve.

A stent according to the invention offers the following advantages. Thestent exerts a constant, gentle radial force on the wall of the lumenthat maintains lumen patency and actively resists compression, as by atumor. The inherent flexibility of the knitted stent adapts toperistalsis, transmitting the peristaltic wave to the lumen, but withoutchanging overall length or creeping. This reduces complications andpromotes long-term stability, patency, and patient comfort. The forceexerted by the stent against the lumen is sufficient to compress thecapillaries of the organ so that growth into the lumen is retarded. Thestent can be delivered via a low-profile delivery system which issmaller than a standard endoscope. The small diameter of the deliverysystem simplifies implantation by eliminating the need for pre-dilatingthe stricture, and allows placement even in patients with tortuousesophageal anatomy or strictures prone to perforation by plastic stents.

In a second aspect, the invention features a stent for providingreinforcement to a selected region of a selected body lumen. The stentcomprises two resilient cylindrical mesh layers and a semi-permeablecompliant membrane sandwiched between.

Preferred embodiments of the invention feature the following. The twomesh layers may be knit of a flexible filament, and the knit may beconfigured so that the stent can adapt to peristalsis of the body lumen.The membrane is composed of expanded polytetrafluoroethylene.

The invention or preferred embodiments thereof may feature the followingadvantages. The semi-permeable membrane prevents cell ingrowth of thestent. The force exerted by the stent against the lumen is sufficient tocompress the capillaries of the organ so that growth into the lumen isretarded.

In a third aspect, the invention features a method of manufacturing adelivery system for a resilient tubular device so that the tubulardevice can be inserted into the body in a substantially reduceddiameter. The method uses a confining block having a bore and a slotleading into the bore. The tubular device is pinched and inserted intothe bore and the slot. Two mandrels are inserted into the bore, oneinside the tubular device and one outside. The mandrels are revolvedabout each other to roll the tubular device on itself until the tubulardevice is entirely rolled and confined at the reduced diameter withinthe bore. The tubular device is removed from the bore while beingrestrained in the reduced diameter.

Preferred embodiments of the method of manufacture feature thefollowing. The removing step may be accomplished by pushing the tubulardevice from the end of the bore and restraining the tubular device as itemerges. The restraining my be by means of wrapping a wire around thetubular device. The slot may be tangent to the bore of the confiningblock. The tubular device may be a stent knit of an elastic filament.One of the mandrels may be part of the delivery system that will be usedto deliver the stent.

The inventive method of manufacturing the stent features the followingadvantages. Certain prior methods required several operators tosimultaneously hold and constrain the resiliency of the stent and hurtthe fingers of the operators. The method of the invention requires onlyone operator and is comfortable to execute. The stent delivery systemsproduced by the method are more uniform than those manufactured byprevious methods, both in distribution of stresses within a single stentand in variation between stents, thus avoiding deformation of the stentduring manufacture and allowing the physician to place the stent moreprecisely in the patient. A stent delivery system manufactured accordingto the method has a small profile, and thus minimizes trauma to thepatient during implantation.

Other advantages and features of the invention will become apparent fromthe following description of a preferred embodiment, and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1c are perspective views of a stent according to theinvention.

FIG. 1a is an end elevation view of the stent.

FIGS. 1b, 1d-1h, 4a, and 6 are detail views of the knitted loops of aknitted stent.

FIGS. 2, 2a, 3, and 3a-3e are sectional views of a body, showing effectsand operation of a stent in the esophagus.

FIG. 4 is a sectional view of a peristaltic organ.

FIGS. 5, 5a, and 5b are schematic representations of alternateembodiments of the stent.

FIG. 5c is a partially broken-away view of an alternate embodiment.

FIGS. 6a, 7, 7a, 7c-7j, 7l, 7m, and 7p-s are perspective views of toolsand a time sequence of steps in a process for manufacturing a deliverysystem for the stent.

FIG. 6b is a perspective view of a stent knit of interlocking pre-formedsinusoidal rings.

FIGS. 7b, 7k, 7n, and 7o are cross-sectional views taken during theprocess of manufacturing the delivery system.

FIG. 7t is a sectional view of the delivery system.

FIG. 7u is a perspective view of the delivery system, cut away.

FIGS. 8 and 8a-8e are a time sequence of sectional views of an esophagusshowing delivery of a stent.

FIGS. 9, 9a, and 9b are a time sequence of cutaway views of an alternatedelivery method.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 1a, a stent 100 according to a preferredembodiment is formed of a knit cylinder with length L and diameter D.The knitting forms a series of loosely-interlocked knitted loops (e.g.,as indicated by adjacent loops 132 and 134 in FIG. 1b) that may slidewith respect to each other. This sliding or shifting allows the stent toadapt to the movement of the organ without moving axially in the organ.The adaptation is accomplished with mere bending of the stent filament.

The stent maintains its axial working length L when locally radiallycompressed, by locally lengthening or shortening due to shifting of therows of loops relative to each other. FIG. 1c shows a region 130 of thestent not under radial compression where adjacent loops 132 and 134 arein an overlapping, relaxed configuration, and the heads of the loops areseparated by a short distance s. In the case of an esophagus, a largepiece of food distends the esophagus. At the first instant of theexpansion, the wall may be deflected by an angle Θ, but the diameter ofthe organ will not have changed appreciably. In such a region, 140 inFIG. 1c, the local length of the wall elongates by a factor 1/cosΘ. Therows of loops of the stent shift axially with elastic deformation of thewire of the loops so that the separation of the heads increases to aloop length l₁, as shown in FIG. 1e. In the region of maximum expansion150, the length of each portion of the esophagus returns to its restlength, but the diameter is extended. The knit loops of the stent canwiden, as shown in FIG. 1f, to accommodate this extension. Returningagain to considering any peristaltic organ, the organ contracts (c ofFIG. 1c) to compress a region. In a region 160 where the wall is at anangle of deflection Θ but the diameter is essentially equal to the restdiameter, the length of the wall of the organ will elongate by a factor1/cosΘ, and the loops will again pass through a state where their widthis essentially the same as the rest width, but, by relative shifting ofthe rows of loops axially, they are extended to length l₁, the stateshown in FIG. 1e. In a region of maximum compression 170, the wall ofthe organ is at its rest length, but the circumference is much reduced.In this region, the loops of the stent deform into the configuration ofFIG. 1g, where the length of the loops is s but the width is compressed.Finally, as the peristalsis relaxes, the wall of the organ returns toits rest length and rest circumference, region 190 of FIG. 1c, and theloops of the stent return to the overlapped rest configuration of FIG.1d.

In the case of organs that can constrict almost closed, as the lumencompresses radially the circumference shortens, and thus part of thelength of the filament that contributes to the circumference of thestent in its rest state is freed to contribute to length, and thus theloops can lengthen to length l₂, as shown in FIG. 1h.

The lengthening from s to l₂ all occurs without significant elongationof the filament of the stent itself, only by elastic bending deformationand sliding of the rows of loops against one other. The ratio of themaximum local length to the relaxed local length, l₂ /s, is determinedby the configuration of the loops and the elastic limit of the materialof the filament.

Referring again to FIG. 1c, the local lengthening in regions of radialextension, compression or slope does not substantially affect the loopsin nearby regions that are not exposed to the radial compression, whichare in turn free to elongate, contract or widen in response to themovements of their own local portions of the organ. Thus, the stentmaintains its overall working length L even when locally extended orcompressed. When the radial compression is released, the elasticity ofthe filament causes the stent to expand back to its original restdiameter D without change of the overall length, since adjacent loops inthe region of compression slide back to the relaxed overlapped state,separated by distance s. Because the stent maintains point-for-pointcontact with the organ, averaged over a local area about the size of oneloop, the stent maintains its placement in the organ, and does notmigrate with the peristalsis.

A further property of the stent is that the state characterized by alladjacent loops being separated by distance s is the stable equilibriumbetween the elastic restoring forces of the wire and the compressiveforce of the esophagus. Thus, the stent automatically adjusts to overalllength L regardless of the initial configuration of the loops and lengthof the stent. For example, if the loops are in extended positions as inFIG. 1e or 1h, then upon compression the loops adjacent the compressedregion draw axially inward to the relaxed configuration in FIG. 1b, thusdrawing the proximal end (120 of FIG. 1) and distal end 122 inward. Whenthe compression releases, the loops in the region of the compressionalso shorten, since the ends 120 and 122 of the stent have been drawninward and adjacent loops slide inward to adjust for the reduced overalllength. Once the stent has settled into this equilibrium state, theoverall length and position within the lumen are stable.

These features are enabled in the preferred stents of the invention by asliding motion of adjacent filament loops, which in turn is enabled bythe method of knitting that reduces overlap of the loops, as shown, andthe elasticity of the filament and the shape of the loops. The slidingmotion allows local axial lengthening or shortening of the stentsubstantially independently of other remotely located portions of thestent. The elasticity of the stent filament allows the stent and lumento return to their desired patency when the compacting force is removed,without inelastic deformation. The loops are configured so that thestent assumes its desired diameter and overall length as the elasticrestoring forces seek their minimum stress, the relaxed state of FIG.1b. This minimization occurs when adjacent loops touch at their widestpoints, as for instance point 136 between loops 132 and 134.

These features are particularly useful in body passages in whichphysiologic function includes motion of the lumen walls, such asperistaltic motion of the esophagus. For example, referring to FIG. 2,an esophagus 200 is occluded by a tumor 202. In FIG. 2a, after insertionof the stent 100, the lumen's patency is restored. Once implanted in theesophagus, the stent is held in a rest diameter that is slightlycompressed by the esophagus from its free diameter when outside thebody. It is the elastic restoring force of the stent resisting thiscompression that holds the stent in place.

A stent for an organ like the esophagus, according to the invention, notonly holds the lumen open but allows the organ to maintain itsphysiologic motion. Further, the stent adapts to this motion withoutitself being subject to the peristaltic force--it does not creep towardthe stomach with each esophageal contraction, nor does the overalllength change. The operation of the elastic knitted stent is illustratedin FIGS. 3 and 3a-3e. A food particle 310 is urged through the lumen 320by a peristaltic wave 322 that propagates down the esophagus 200. Thewave is induced by circumferential contraction of the muscular tissuesurrounding the lumen, a consequence of which is radial inward extensionof the wall. Before the wave reaches the stented portion, the stent liesbetween points T and B, with length L between. As the wave reaches pointT and the portion of the esophagus reinforced by the stent 100, thestent complies with the radial contraction, as shown in FIGS. 3a-3e. Asshown in FIG. 3e, after passage of the peristaltic wave the stent hasnot migrated from points T and B, and maintains its overall length L.The adjustment and restoration feature allows the stent to maintain itsposition in the lumen, without migrating axially as might occur with aunitary structure in which stresses in one portion are substantiallytransmitted to other portions.

Referring to FIGS. 4 and 4a, the configuration of the knit loops of thestent may be determined based on the degree of radial motion andconsequent axial lengthening imposed by the peristaltic motion.Generally, the loop length of the stent in its extended position l canbe expressed as:

    l=εs                                               (1)

where s is the axial length of the portion of the body lumen over whichtwo adjacent loops of the stent extend, and ε is the factor by which thestent must elongate in response to local lengthening of the body lumenwall. It will be seen that the maximum local lengthening will occur overthe portion of the wall that lies at the largest angle Θ from theat-rest position. In a worst-case limit, the entire peristaltic wave canbe approximated as having a wall at angle Θ, thus distending a portionof the lumen that has rest length a to a triangular wave with hypotenuseb. Thus,

    b/a=1/cos Θ                                          (2)

This ratio, b/a, is the elongation factor by which the loops mustlengthen from their relaxed length s to their extended length l toaccommodate the lengthening of the lumen wall at the incline of theperistaltic wave. Thus,

    b/a=1/cos Θ=l/s=ε                            (3)

For the stent as a whole to maintain point-for-point contact with thewall of the lumen and thus for the stent as a whole to remain stationaryalong the axis of the lumen, the heads of the loops are allowed to slidelocally in the region of peristaltic compression by a distance (l-s)/2.

It will be noted that the stent will accommodate the elongation of thelumen wall if it is capable of local elongation ε equal to 1/cosΘ,independent of the amount of deflection c, even in the extreme casewhere the organ is capable of constricting completely shut--deflection cin FIG. 4a being equal to the radius D/2 of the lumen.

The amount of force exerted by the stent against the lumen wall ischosen to exceed the blood pressure within the capillaries of a typicaltumor, and thereby prevent the tumor from further growth into the lumenof the esophagus. The exerted force is determined by the coefficient ofelasticity of the filament, by the configuration of the loops anddensity of the knit (loops per unit of axial length), and by thediameters of the lumen and the stent. For instance, a stent design wouldexert more force against the lumen wall by choosing a stiffer materialfor the filament, by knitting the stent with more, smaller loops perunit of length (decreasing s, and reconfiguring the loops to maintainthe ratio l/s), or by knitting the stent to a larger rest diameter D.The radial force exerted is bounded at the point where the loops reachthe relaxed configuration of FIG. 1b and the stent's diameter reachesdiameter D--the forces sum to zero at the contact points 136, and theforce exerted on the lumen itself falls to zero. Thus, the diameter D ofthe stent must be slightly larger than the diameter of the lumen if thestent is to retain its place in the lumen.

Referring again to FIGS. 1, 1a and 1b, a particular embodiment for useas an esophageal stent is knitted of nitinol wire of about 0.15millimeters in diameter to have a diameter D of about 18 mm, thoughdiameters of 14 mm to 25 mm may be useful. The proximal end 120 isflared to 20 mm, to secure fixation to the esophageal wall. Stentsmanufactured in overall lengths varying from 5 to 15 cm allow selectionof a stent tailored to the patient's needs. The relaxed loop length s isabout 0.80-0.85 mm, and the maximum loop length l attained withoutsignificantly distorting the loops is about 1.05-1.15 mm. Thiselongation factor of about 1.4 is close to √2, allowing for a maximumangle Θ of about 45°. The peak-to-peak height p of the loops is about2.2 mm when the loops are in their rest state.

Examples of filament materials include shape memory metals, for example,nitinol, tantalum steel, stainless steel or other elastic metals, orplastics such as polyester, polypropylene, or carbon fiber. The filamentmay be selected to have a sufficiently high elastic limit to allow thedelivery system to rely entirely on this elasticity, rather than, forexample, the balloon 820 of FIG. 8e to expand the stent. The filamentmay be formed of a two-component metal wire system that exhibitsdesirable physical properties such as enhanced radiopacity along withdesirable mechanical properties, such as extreme elasticity. A fulldiscussion of composite medical wires will be found in U.S. applicationSer. No. 07/861,253, "Medical Wire" by Kevin R. Heath, incorporatedherein by reference. The stent may be knit of two or more filaments.

As shown in FIG. 4a, the stent is knitted of a single filament. Thisview shows only the front half of the stent. The loops that appear inthis view as independent rows in fact are a single spiral-wrappedsequence, much as a common screw has only one ridge from head to tip. Inalternate embodiments of the stent, multiple filaments or other knitscould be used, so long as the knit structure allows a single row toelongate without forcing the two adjacent rows to shift. The last loopof the wire is cut 440. To prevent the stent from unravelling, the lastthree loops (two shown, 450 and 452) of the stent are coated inurethane, as shown in FIG. 4b. The coating also covers the sharp ends ofthe filament.

It will be realized that the stent is applicable to malignant or benignobstructions of many other organs. Stents to treat biliary ductobstructions, for instance when treating liver sclerosis or bleeding,would be about 8 to 10 mm in diameter and 4 to 8 cm in length. Stentsfor the ureter would be about 6 to 10 mm in diameter and about 2 to 10cm in length. Urethral stents would be about 10 to 20 mm in diameter andabout 2 to 6 cm in length. Stents for the prostatic urethra would beabout 10 to 20 mm in diameter and about 2 to 6 cm in length. Colonicstents would be about 10 to 20 mm in diameter and about 4 to 10 cm inlength. Stents for hemodialysis shunts would be about 6 to 8 mm indiameter and about 2 to 6 cm in length. Stents for the porta canal wouldbe about 8 to 14 mm in diameter and 4 to 8 cm in length. Stents for thetrachea and bronchi would be about 8 mm to 25 mm in diameter and 1 to 8cm in length. Stents for gastric outlet obstructions would be about 8 to20 mm in diameter and 1 to 25 cm in length. Peristaltic stents may alsobe configured for aortic aneurysms or dissections (preferably weavingthe filament material with a covering such as dacron), and treatment ofsuperior vena cava syndrome and venous restrictions. The invention isalso useful in lumens in which compression is caused by some outsideforce, for example in blood vessels compressed by muscular contraction,movement of an extremity, or pressure exerted by an object external tothe body.

FIGS. 5, 5a, and 5b represent alternate forms for stents. (Showing theknit loops in these projections would obscure the shape; these figuresrepresent only shapes of the stents.) The stent could be shaped toinclude, in its free and rest states, a narrowing at a point in itslength. This narrowing would accommodate the stent to the anatomy of anatural sphincteric structure, for instance the pylorus or the cardia. Astent so narrowed would enable the organ to close, for instance toprevent reflux. The narrowing could be shaped at one of the ends, forinstance for use in the rectum at the anus or in the common duct for aPapilla of Vater.

The narrowing could be shaped conically, as in FIG. 5 for use insphincter organs. FIG. 5a shows a stent incorporating a flattening, withthe circumference in the area of the flattening reduced so that thewidth remains constant. The latter embodiment could be used in anocclusion with two lips such as the vocal cords. In either case, it maybe desireable to form the loops in the region of the constriction sothat their free state is similar to one of the compressedconfigurations, e.g. FIG. 1g, so that the constriction is capable ofopening to the rest diameter D.

As shown in FIG. 5b, the margins of the narrowing could incorporate avalve to ensure complete closure of the stented organ, as for instancethe reinforced lips 520. This valve could be opened and closed either bythe muscles normally surrounding the point of constriction or by amanually-operated control extended to outside the body. This would allowthe use of the stent, for instance, across the aortic valve or as areplacement for the urinary sphincter. It may be desireable to reinforcethe point of the constriction, e.g., with a stiff wire, especially inconjunction with the flattened constriction of FIG. 5a. It may also bedesireable to provide a valved stent with a watertight membrane, similarin form to that discussed below and shown in FIG. 5c.

The stent can be made to exert varying force along its length, forinstance by varying the gauge of the wire or the density of the knit. Inthe narrowed stents discussed above, it may be desireable to make thestent particularly flexible in the region of the narrowing.

Some tumors are so invasive that the stent is quickly ingrown by thetumor. As shown in FIG. 5c, the stent may be manufactured with anelastic semi-permeable membrane 530 of porosity less than 50 microns andof very low modulus of elasticity, sandwiched between two knit layers.The membrane may advantageously be of expanded polytetrafluoroethylene(teflon) or latex. The inner layer 532 is essentially identical to thesingle-layer stent, providing most of the elastic force against thelumen. The outer knit layer 534, which acts to retain the membrane, istypically constructed of thinner wire, for instance 0.07 mm diameter, ora less-resilient material such as polypropylene or polyethylene. Theouter knit layer is slightly shorter in length than the inner layer.

The stent is knit on a conventional knitting machine, very similar tothat used to knit stockings. During the knitting process, the wire isdeformed past its elastic limit. Referring to FIG. 6, on some knittingmachines, or for stents of some diameters, it may be convenient toproduce a knit with the "up loops" 610 of a different shape than the"down loops" 612. In some applications, for instance the aorta, it isimportant that the loops be uniform so that the stent exerts uniformpressure along the lumen wall. During the knitting process, the wirewill be under tension, and thus the loops will be in a tightconfiguration, similar to FIG. 1e, or possibly to FIG. 1f or 1hdepending on the geometry and setup parameters of the knitting machineitself.

The knitting machine produces a long "rope" of knit loops. The rope iscut into lengths somewhat longer than the final length of the stent. Theextra length allows for the shortening of the stent that will occur asthe loops are shortened from the elongated state in which they emergefrom the knitting machine to the rest state of FIG. 1b, and allows forsome trimming. As shown in FIG. 6a, after knitting, the stent is mountedon a mandrel 620 for annealing, to relieve the strains induced by theplastic deformation of knitting and to produce greater elasticity in thewire. The mandrel is in the free shape of the stent, 18 mm in diameterwith a flare 622 to 20 mm at one end. To achieve the constrictedembodiments of FIGS. 5 and 5a, the mandrel would have a constrictionformed in it, and an external restraint would be applied to the stent sothat the annealed shape would be as shown in those figures. As the stentis loaded onto the mandrel, the operator shortens the overall length sothat the loops assume the relaxed, shortened state of FIG. 1b. The stentis annealed at approximately 450° C. for about 15 minutes.

After annealing, the stent is cut to its final length, and the threeloops at each end of the stent each receive a drop of urethane (450 ofFIG. 4a or FIG. 6) to prevent unravelling.

Alternately, the stent may be knit of interlocking pre-formed sinusoidalrings, two of which 630, 632 are shown in FIG. 6b.

The stent itself is packaged into a delivery catheter as shown in FIGS.7 and 7a-7u. The center of the delivery catheter is a carrier tube 700,shown in FIG. 7. The carrier is a flexible tube of Pebax, apolyether/polyamide-12 resin from Atochimie with desireableflexibility/rigidity characteristics, 2.5 mm in diameter andapproximately 80 cm long. The carrier has several radiopaque O-rings704,706 mounted along the most-distal 20 cm. The preferred radiopaquematerial is tantalum.

The preferred process of mounting the stent on the carrier tube 700 usesseveral tools: a confining block, two mandrels, and a pusher. Theconfining block 710, shown in FIGS. 7a-c, is cylindrical, somewhatlonger than the stent itself at about 20 cm, and formed of a rigidplastic with low friction characteristics, preferably delrin or nylon.The block has a drilled bore 712 of 8 mm diameter, with a 1 mm-wide slot714 cut from the top of the outside of the block and meeting the innerbore 712 at a tangent. The slot and bore may have a guideway 718 formed,to make the following steps easier. The block may also have flats 716milled in the bottom so that the block may be mounted in a vise. A firstmandrel 720, shown in FIG. 7d, is a simple rod approximately 30 cm longand about 3 mm in diameter. A second mandrel 722, shown in FIG. 7e, hasa shaft of about 3 mm diameter and length longer than the confiningblock, two handles 724, each about 10 mm in diameter, with center boresthat friction fit on the ends of the mandrel shaft, and slots 726 ofwidth to accommodate the carrier tube. Both mandrels have rounded endsso that they will not catch on the loops of the stent. A third tool is apusher 728, shown in FIG. 7f, with a shaft 729 of slightly less than 8mm diameter and a bore 730 somewhat larger than the outer diameter ofthe carrier 700. The bore may either be the full length of the pusher,or as shown in FIG. 7f, may have a breech hole 732. A fourth tool, whichwill be seen in FIG. 7q, is a soft copper wire with a silicone sheath760 (silastic) over it. The sheathed wire is about 50 cm long, and thesheath is about 1-2 mm in diameter.

Referring to FIG. 7g, the confining block 710 is securely mounted, as ina vise 750. An operator squeezes a stent 100 flat and works it into theslot 714 preferably starting at a corner 752, and bottoms it in the bore712. Referring to FIG. 7h, the stent is positioned in the confiningblock so that the proximal end 120 of the stent extends from the end ofthe confining block. The first mandrel 720 is inserted into the distalend of the carrier 700, and then the mandrel and carrier tube are passedthrough the center of the stent. The operator slides the stent to thecenter of the confining block, as shown in FIG. 7i. Referring to FIG.7j, the stent is slid back so that the flared proximal end again extendsfrom the end of the confining block. One handle of the second mandrel isremoved, and the shaft 722 of the second mandrel inserted through thebore of the confining block but outside the stent. Referring to FIG. 7k,the first mandrel 720 lies inside the carrier tube 700, which in turnlies inside the stent 100. The lower portion of the stent and the secondmandrel 722 lie inside the bore 712 of the confining block. Referring toFIG. 7l, the stent is slid back to the center of the confining block.The several slides forward and back equalize the distribution of theknit loops evenly over the length of the stent. The second handle of thesecond mandrel is affixed to the shaft of the second mandrel, and theslots 726 of the handles engaged with the carrier tube 700 and/or firstmandrel 720. The operator can center the O-rings 704,706 within thestent so that the carrier will be axially positioned roughly correctlywithin the stent. Referring to FIG. 7m, the operator twists the handles,revolving the two mandrels about each other and winding the stent aboutthe two mandrels. FIG. 7n shows the configuration of the stent and thetwo mandrels after about half a revolution. The operator continueswinding until the stent is completely rolled on itself in the bore ofthe confining block. The operator removes a handle from the secondmandrel and removes the second mandrel from the confining block. Thestent is held in the wound conformation by the confining block, as shownin FIG. 7o.

Referring to FIG. 7p, the operator threads the pusher 728 over theproximal end of the carrier, with the shaft 729 distal. The pusher willbe used to slowly push the stent out of the bore of the confining block.

Referring to FIG. 7q, using the pusher the stent is pushed out of theconfining block by about 1 cm. The operator makes any final adjustmentsrequired to center the radiopaque O-rings within the stent. The operatorwraps several turns of the copper wire and silicone sheath 760 aroundthe exposed distal end 122 of the stent, with about a 1 mm gap betweenturns, and lays the bight of the sheathed wire into the slot of theconfining block. Referring to FIG. 7r, the operator gradually feeds thestent out of the confining block using the pusher, and wraps thesheathed wire around the stent to keep it confined to a diameter ofabout 8 mm. The operator maintains a roughly uniform 1 mm spacingbetween wraps.

After the stent is fully bound in the copper wire 760, the pusher isremoved back over the proximal end of the carrier tube and the carriertube is pulled out of the bore of the confining block, the stent andsilastic/copper wrap is dipped in U.S.P. grade dissolving gelatin, andthe gelatin is allowed to set. The copper wire can then be unwrapped;the silicone sheath acts as a release surface so that the gelatin peelsoff the wire and remains set on the stent in a 1 mm-wide "threaded"strip, 770 in FIG. 7s, confining the stent.

Referring to FIGS. 7t and 7u, the stent delivery system catheter 799 iscompleted by affixing a nose piece 772 onto the distal end of thecarrier 700, and surrounding the entire assembly in a cylindrical sheath774. Both the carrier and sheath are essentially rigid in the axialdirection, so that they can be used to push or pull the catheter toposition it, and so that handles 782 and 784 can be squeezed together toretract the sheath from over the stent. Also shown in FIG. 7t are theradiopaque markers 704,706 and in FIG. 7u graduation markings 778 on thesheath, both of which will be used during implantation to guidepositioning. The inner pair of the markers indicate the length the stentwill assume at its 18 mm fully-expanded diameter, and the outer pairindicate the length of the stent when compressed to 8 mm diameter. Aguidewire 778 will be threaded through the center bore 776 of thecarrier during implantation.

The process of implanting the stent is illustrated in FIGS. 8 and 8a-8e.Referring to FIG. 8, using an endoscope 810, the proximal margin 812 ofa stricture 814 is identified. The guidewire 778 is advanced across andbeyond the stricture. In FIG. 8a, an 8 cm-long balloon 820 is advancedover the guidewire and inflated to 12 mm diameter, dilating thestricture to 12 mm. After examining the stricture endoscopically andfluoroscopically, a gelatin-encased stent 4-6 cm longer than thestricture is chosen. The delivery system 799 is passed over theguidewire and advanced until the distal inner radiopaque marker 704 is2-3 cm below the distal margin 832 of the stricture.

Referring to FIG. 8c, The outer sheath is retracted by squeezingtogether handles 782 and 784 (see FIG. 7u), and the stent begins todeploy. The gelatin will immediately begin to dissolve, allowing thestent to expand under its own elastic restoring force. The material ofthe stent filament, nitinol, is chosen so that even the fairly severedeformation required to compact the stent into the delivery system doesnot exceed the elastic limit. Referring to FIG. 8d, after the proximal120 and distal ends 122 of the stent have expanded and firmly attachedto the esophageal wall, the catheter 799 can be removed. Referring toFIG. 8e, depending on the patient, a 12mm-diameter balloon 820 may beinflated within the stent to ensure that the occlusion is opened to thedesired patency, to affix the stent firmly to the esophageal wall, andto ensure adequate esophageal lumen size for endoscopic examination.Peristaltic contractions of the esophagus will allow the stent to"settle" into its most-relaxed configuration.

Referring to FIG. 9, in another delivery system, the stent 100 is formedof an elastic filament material that is selected so compaction producesinternal restoring forces that allow the stent to return to its restdiameter after the compacting restraint is removed. The stent may becompressed onto a catheter 900 that includes a sleeve 902; the sleeveholds the stent in a relatively compacted state. The compaction istypically achieved by rolling the stent upon itself using two mandrels,as in FIGS. 7g-7o. In other cases, the stent may be positioned coaxiallyover the catheter. The catheter is positioned within the lumen at theregion of the tumor 202. In FIG. 9a, the sleeve is removed from aboutthe stent, for example, by withdrawing axially in the direction of arrow910, thus allowing the stent 100 to radially expand by release of itsinternal restoring force. As shown in FIG. 9b, the axial force exertedby the stent is sufficient to dilate the lumen 200 by pushing the tumorgrowth 202 outward, or in some cases to compress the occlusion againstthe lumen wall. The catheter can then be removed.

Other embodiments are within the following claims.

What is claimed is:
 1. A unitary stent for providing reinforcement to aselected region of a selected body lumen, comprising:a resilientcylindrical supporting mesh inner layer; a resilient cylindricalretaining mesh outer layer of inner diameter about equal to the outerdiameter of said inner layer; and a semi-permeable compliant membranesandwiched between and secured in position by said supporting mesh innerlayer and said retaining mesh outer layer, to form said unitary stent,said semipermeable membrane being sufficiently impermeable to preventingrowth of cells into said stent.
 2. The stent of claim wherein saidmembrane is composed of expanded polytetrafluoroethylene.
 3. The stentof claim 1 wherein said inner and outer layers are knitted of metal wiresuch that said stent is self-expandable.
 4. The stent of claim 3 whereinsaid metal wire is nitinol wire.
 5. The stent of 3 wherein said metalwire has a diameter of about 0.15 mm.
 6. The stent of claim 1 whereinsaid stent has a diameter of about 14 to 25 mm and a length of about 5to 15 cm.
 7. The stent of claim 1 wherein said stent has a diameter ofabout 8 to 10 mm and a length of about 4 to 8 cm.
 8. The stent of claim1 wherein said stent has a diameter of about 6 to 10 mm and a length ofabout 2 to 10 cm.
 9. The stent of claim 1 wherein said stent has adiameter of about 10 to 20 mm and a length of about 2 to 6 cm.
 10. Thestent of claim 1 wherein said stent has a diameter of about 8 to 12 mmand a length of about 2 to 6 cm.
 11. The stent of claim 1 wherein saidstent has a diameter of about 10 to 20 mm and a length of about 4 to 10cm.
 12. The stent of claim 1 wherein said stent has a diameter of about12 to 25 mm and a length of about 4 to 10 cm.
 13. The stent of claim 1wherein said stent has a diameter of about 6 to 8 mm and a length ofabout 2 to 6 cm.
 14. The stent of claim 1 wherein said stent has adiameter of about 8 to 14 mm and a length of about 4 to 8 cm.
 15. Thestent of claim 1 wherein said stent has a diameter of about 8 to 25 mmand a length of about 1 to 8 cm.
 16. The stent of claim 1 wherein saidstent has a diameter of about 8 to 20 mm and a length of about 1 to 25cm.
 17. The stent of claim 1 wherein said lumen has a wall that issubject to peristaltic motion in which localized radial contraction ofthe lumen progresses in a wave-like manner along the lime, said lumencharacteristically exhibiting transient localized lengthening of thelumen wall in an amount that conforms to a profile of the peristalticcontraction wave, and wherein said inner and outer layers are formed ofa filament extending in a pattern of axially adjacent interlocked loops,the pattern and arrangement of said loops selected such that from arelaxed state of said loops, each of said loops is permitted to shift inthe axial direction of the stent relative to and independently of loopsdistal thereof and proximal thereto, to permit transient localizedlengthening of the stent with localized lengthening of correspondingportions of said lumen wall, the amount of said permitted shifting ofsaid loops being sufficient to accommodate said wave profile withoutdisturbing loops that engage other, at-rest portions of the lumen walldistal of and proximal to said wave profile, said shifting to permitlocalized stent lengthening being accommodated by bending of saidfilament, said permitted localized stent lengthening being by a factor(ε) defined by the ratio ε=l/s, where l is the maximum extended lengthof said portion of said lumen that corresponds to said wave profileduring contraction and s is the wall length of that portion of saidlumen when at rest, whereby said stent can locally lengthen and shortenwith the physiologic motions of said body lumen during peristalsis andthereby resist migration from said selected region.